A Yemeni soldier inspects a car at a checkpoint on a street leading to the U.S. Embassy in Sanaa on Aug. 4. / Hani Mohammed, AP

by Oren Dorell and David Jackson, USA TODAY

by Oren Dorell and David Jackson, USA TODAY

The shutdown of U.S. embassies and a warning to American tourists came in response to an intercepted message between top al-Qaeda figures about a possible weekend attack, U.S. officials said. Some terrorism analysts say the plot may still be active.

News organizations reported that electronic communications were intercepted between Ayman al-Zawahri, who succeeded Osama bin Laden as the head of al-Qaeda, and Nasir al-Wuhayshi, the head of al-Qaeda in the Arabian Peninsula (AQAP).

The intercept was significant and surprising, because it's rare that top al-Qaeda leaders are caught discussing a specific plot or the timing, officials said. Zawahri was in Pakistan and Wuhayshi in Yemen, but the intercepts did not reveal the specific location or target.

The United States shut down 20 embassies and consulates in the Middle East and Africa through Saturday. The Obama administration's decision may have disrupted an al-Qaeda terrorist plot, but that doesn't mean the terrorists will abandon their plans for good, a terrorism expert says.

"These types of plots - they get disrupted, and sometimes the hit squads are called up. Sometimes they get folded into another operation in the future," said Thomas Joscelyn, a terrorism analyst at the Foundation for the Defense of Democracies. In the long term, however, "al-Qaeda doesn't give up on plans," Joscelyn said.

The Oct. 12, 2000, suicide bombing of the USS Cole occurred after a failed attempt against the USS Sullivans in January of that year. The destruction of the twin towers of the World Trade Center in 2001 came after a previous attempt to destroy one of the World Trade Center buildings in 1993, with a truck bomb.

"When they have plans on the books, if they think they're good plans, they come back to them," Joscelyn said.

The State Department said the closures of embassies and consulates were out of an "abundance of caution" and not an "indication of a new threat." White House spokesman Jay Carney said the United States faces "an ongoing threat from al-Qaeda and its affiliates," and he singled out AQAP.

Carney said that although al-Qaeda has been "diminished" as a global force, "affiliate organizations ... have strengthened."

In the United States, New York City said it was increasing security at some landmarks and houses of worship in response to he warnings. Some airports, such as Boston's Logan Airport, boosted security, but other major airports, including Atlanta and San Francisco, said they have not.

The Transportation Security Administration had not implemented or requested new security measures, according to airline officials.

For the United States to shut down so many diplomatic facilities over such a large geographical area means U.S. intelligence officials believe the plans they came across "had some likelihood of success, meaning there's been surveillance, reconnaissance, (and) teams have infiltrated to where they need to be," says Mark Jacobson, a senior fellow at the German Marshall Fund.

"So, if it's this serious, this is something that's been in the works for a while," said Jacobson, former deputy NATO representative in Afghanistan from 2009 to 2011.

Exactly who remains behind the threat is still unclear, he said. It could be a resurgence of core al-Qaeda in Pakistan, one faction gaining dominance, or coordination between various factions, he said.

The New York Times reported Monday that the closures were in response to intercepted communications between Zawahri and Wuhayshi.

At the request of senior U.S. intelligence officials, the Times said it had withheld their identities from an article posted online Friday and published Saturday. However, McClatchy Newspapers disclosed the names Sunday, and the Times then followed suit after the Obama administration dropped its objections.

"This was significant because it was the big guys talking, and talking about very specific timing for an attack or attacks," one unnamed U.S. official told the Times.

Citing a U.S. intelligence official and a Middle East diplomat, the Associated Press reported Monday that Zawahri's message was picked up several weeks ago. The threat appeared initially aimed at targets in Yemen but was expanded to include U.S. or other Western sites abroad.

Intelligence sources told NBC News that Zawahri and Wuhayshi "wanted to do something big" this past Sunday, an Islamic holiday commemorating the 27th night of Ramadan and the day the Quran was revealed to the prophet Mohammed.

CBS News carried a similar report, also pegged to unidentified sources, that the pair were discussing "something big" connected to Ramadan, which ends Thursday.

AQAP has been linked to several terrorist plots, including the attempted Christmas Day 2009 bombing by Umar Farouk Abdulmutallab of a U.S. airliner as it approached Detroit. The bomb, hidden in his underwear, failed to detonate.

It worked with the man charged in the Times Square bombing attempt and on a plot to bring down airliners with bombs in printer cartridges, both of which happened in 2010. Deceased American al-Qaeda cleric Anwar al-Awlaki was part of AQAP in Yemen.

The new embassies ordered to shut down through Saturday include facilities in central and east Africa, including the capitals of Burundi and Rwanda. Authorities also closed a third site in Saudi Arabia. Among those closed Sunday: posts in Afghanistan and Iraq.

Security was beefed up in Cairo and elsewhere as U.S. embassies and consulates closed because of the terror threat.

Carney said the United States is focusing more attention on AQAP, which announced a new second in command as the terrorist network launched a raft of prison outbreaks that freed hundreds of al-Qaeda foot soldiers and senior operatives.

Prison breaks last week in Iraq and Libya appear al-Qaeda-related and one in Pakistan was perpetrated by the Pakistani Taliban, which is closely linked to al-Qaeda, Joscelyn said. Hundreds of inmates were freed in each, including many al-Qaeda operatives, he said.

Together the news looks like a "coming-out party" for al-Wuhayshi, the new chief of AQAP, whom Zawahiri last week named his second in command, Joscelyn said. Al Wuhayshi, who had been a deputy of Osama Bin Laden since before 9/11, was himself broken out of a Yemeni prison in 2009.

"Under his leadership of AQAP, he was able to seize turf in Yemen while simultaneously launch plots against the United States," Joscelyn said.

Al Wuhayshi's rise in the organization shows that Obama administration officials, such as CIA Director John Brennan, were wrong to try to distinguish between "al-Qaeda central" and the movement's affiliates in other countries, Joscelyn said.

"If this guy is now general manager of al-Qaeda operations and its affiliates - this shows (that) just because he's in Yemen doesn't make him any less of a core al-Qaeda member," Joscelyn said.

A worldwide travel alert remains in effect until Aug. 31. That alert warns U.S. citizens that al-Qaeda and affiliated groups continue to plan terrorist attacks particularly in the Middle East and North Africa.

"We may be seeing an effort, a trend, of al-Qaeda trying to announce its relevance, trying to show the world it's still in the game," said Frederic Wehrey, a senior associate in the Middle East Program at the Carnegie Endowment for International Peace. "And some sort of spectacular attack on a U.S. facility would certainly do that."

This week marks the 15th anniversary of terrorist attacks on U.S embassies in Nairobi, Kenya and Tanzania that killed hundreds of people. France, Germany and the United Kingdom also closed their embassies in Yemen on Sunday, the start of the workweek in the region.

Contributing: Charisse Jones in New York; Sarah Lynch in Cairo; the Associated Press